This invention relates in general to case-hardened steel and more particularly to a process for determining the case depth of case-caburized steel.
Low carbon steel has good ductility and as such will withstand bending stresses and impacts quite well. However, it cannot be hardened to the extent required for wear-resistant surfaces. High carbon steel, on the other hand, by reason of its higher carbon content, will transform into a large proportion martensite when subjected to a proper heat treatment--and martensite is the hardest structure that can be obtained from steel in any appreciable amount. A properly hardened high carbon steel resists fatigue, wear, indentation and abrasion, and as such provides a good wear surface. But high carbon hardened steels are somewhat brittle and certainly not as tough as low carbon steels. Case-carburizing enables ductile low carbon steel to acquire a hard surface or case which resists fatigue, wear, indentations and abrasion. Thus, case-carburized steel possesses the attributes of both low carbon steel in the core region and properly treated high carbon steel in the case region.
Iron at elevated temperatures on the order of 1350.degree. F. to 1850.degree. F. exhibits an affinity for carbon, so when a workpiece formed from low carbon steel is heated in a carbon-rich atmosphere, the carbon diffuses into the steel. The extent of the diffusion depends on the constituency of the carbon-rich atmosphere, which is often carbon monoxide and methane, the temperature to which the steel is heated, and the time it remains in the carbon-rich atmosphere. In effect, the region at the surface of the steel workpiece transforms into high carbon steel. Thus, when the workpiece is heated above the temperature at which the carbon-enriched portion becomes austenite, and then quenched, the carbon-enriched portion to a large measure transforms into martensite and becomes a hard case, but the remaining portion, called the core, remains relatively soft and ductile.
One of the more important applications of case-carburizing resides in the manufacture of roller bearings, particularly tapered roller bearings. The races of these bearings must withstand impact stresses and thus should have the ductility of low or medium carbon steel. However, the surfaces of the races, particularly the surfaces which the rollers contact, should be hard to resist wear, indentations and abrasion. Case-carburizing further imparts residual compressive stresses to the cases of the ring-shaped races and this enables the races, along their raceways to better withstand bending fatigue and to inhibit the propagation of cracks from nicks.
Of course, the case cannot be too shallow; it must have reasonable depth to perform its function. But measuring case depth has heretofore been a time-consuming procedure requiring destruction of carburized specimens--and one should know whether the case of a workpiece meets minimum requirements. Actually, no distinct interface exists between the case and the core. Instead, the amount of carbon diffused into the steel simply decreases with depth to the point that the carbon content remains constant at that of the core. Typically, metallurgists use the depth at which a selected carbon content, such as 0.5% carbon, exists as the depth of the case. With the term "case depth" so defined, two procedures have been developed for ascertaining it--at least in connection with ring-like workpieces such as bearing races--namely, the Ms (Martensite start) procedure and carbon gradient procedure. Both require destruction of a specimen and are practiced only on carbon cut rings which are placed in the carburizing furnace with actual workpieces, the assumption being that the carbon cut rings, which are formed from the same steel as the workpieces, will absorb as much carbon as those workpieces and hence acquire a case of the same depth. Apart from being destructive, the tests are also very time-consuming.
The Ms procedure relies on the capacity of steel, when heated to austenite and subsequently quenched, to form martensite which has a well-defined crystalline structure that is readily apparent under a microscope. Actually, the transformation from austenite to martensite begins at a so-called Ms (Martensite start) temperature and that temperature varies with carbon content. For example, the Ms temperature for steel having 0.8% carbon by weight is lower than the Ms temperature for steel having 0.5% carbon by weight. Thus, when the carbon cut ring is heated above its austenizing temperature and then quenched to the Ms temperature for 0.5% carbon, all steel within the case containing that proportion of carbon or less transforms into martensite. The ring remains at the Ms temperature for a short period of time and then is quenched in water. With this quench the steel which has more than 0.5% carbon, becomes "fresh" martensite. Once the ring is cut and the cross-sectional surface polished and etched, the boundary between the original martensite and the fresh martensite stands out quite clearly. One can of course measure from that boundary to the surface of the ring to obtain the case depth, that is the depth at which 0.5% carbon concentration exists. However, this procedure is subjective in nature.
In the carbon gradient procedure, the carbon cut ring is secured in the chuck of a lathe and chips are removed at various depths, with the depth of each chip being recorded. The chips are then subjected to chemical analysis for carbon content. The depth recorded for the chip which shows 0.5% carbon content represents the case depth.
To be sure, others have experimented with nondestructive procedures for determining case depth, but have met with only limited success and no industry-wide procedure has evolved from any of the work. For example, some have attempted to measure the back scattering from acoustic waves sent into a workpiece, but the absence of a well-defined boundary between the case and core in case-hardened steel prevents this technique from being of much value. Others have used surface waves at various frequencies to plot dispersion curves for different case depths, but these efforts have not resulted in a meaningful testing procedure suitable for industry. The objective of these studies has been to study the depth profiles of elastic properties and hardness below the surface, not to measure case depth in a case-carburized specimen. Besides, the problem resides in the transducers used to impart the wave and detect its presence, for they, being piezoelectric devices, must be physically coupled to the workpiece under study.
The present invention resides in measuring the velocity of an acoustic wave which passes through the case of a specimen, and comparing that velocity with an existing correlation of velocity and case depth to determine the case depth corresponding to that velocity, which is the case depth of the specimen.